Known sequential combustion gas turbines can include a first burner, wherein a fuel is injected into a compressed air stream to be combusted and generate hot gases that are partially expanded in a high pressure turbine.
The hot gases coming from the high pressure turbine are then fed into a reheat burner. Fuel is injected into the reheat burner to be mixed and combusted in a downstream combustion chamber. The hot gases generated are then expanded in a low pressure turbine.
FIGS. 1-3 show an example of a known reheat burner.
With reference to FIGS. 1-3, known burners 1 can have a quadrangular channel 2 with a lance 3 housed therein.
The lance 3 has nozzles from which a fuel (for example, gaseous fuel or liquid fuel, such as oil) can be injected. As shown in FIG. 1, the fuel can be injected over a plane known as an injection plane 4.
A channel zone upstream of the injection plane 4 (in the direction of the hot gases G) is a vortex generation zone 6. In this zone vortex generators 7 are housed, projecting from walls of the channel 2 to induce vortices and turbulence into the hot gases G.
A channel zone downstream of the injection plane 4 (in the hot gas direction G) is a mixing zone 9. This zone has plane, diverging side walls 10, and defines a diffuser with an opening angle A relative to a channel longitudinal axis typically below 7 degrees, to avoid flow separation from an inner surface of the side walls 10.
As shown in the figures, over a total channel length, the side walls 10 of the channel 2 may converge or diverge to define a variable burner width w (measured at mid-height), whereas the top and bottom walls 11 of the channel 2 can be parallel to each other, to define a constant burner height h.
The structure of the burner 1 is arranged in order to achieve a compromise of hot gas velocity and vortices and turbulence within the channel 2 at the design temperature.
A high hot gas velocity through the burner channel 2 can reduce NOx emissions (because the residence time of burning fuel in the combustion chamber 12 downstream of the burner 1 can be reduced) and increases the flashback margin (because it can reduce the residence time of the fuel within the channel 2 making it more difficult for the fuel to achieve auto ignition) and can reduce water consumption in oil operation (water is mixed with oil to reduce the likelihood of flashback).
In contrast, high hot gas velocity can increase the CO emissions (because the residence time in the combustion chamber 12 downstream of the burner 1 is low) and pressure drop and increase efficiency and achievable power.
In addition, a high vortex and turbulence degree can reduce the NOx and CO emissions (due to good mixing), but can increase the pressure drop and reduce efficiency and achievable power.
In order to increase the gas turbine efficiency and performances, the temperature of the hot gases circulating through the reheat burner 1 can be increased.
Such an increase causes the equilibrium among all the parameters to be missed, such that a reheat burner, operating with hot gases having a higher temperature than the design temperature, may have flashback, NOx, CO emissions, water consumption and pressure drop problems.